Many powdered and granular solids are produced by a grinding and/or calcining process. The heat generated by this process is transferred to the solid causing a rise in temperature. In many instances it is undesirable to use or store a solid at elevated temperatures.
For instance, in the concrete industry the temperature of the poured concrete is often critical. In general, the cooler the concrete when poured, the stronger it will be. Many construction projects require that the concrete be poured at or below a certain specified temperature. This is especially true during the summer months for large construction projects such as building dams and foundations for fossil fuel and nuclear power plants. The concrete temperature can be controlled by controlling the temperature of its components--cement, aggregate and water.
Currently, concrete is cooled by cooling the water prior to its use in the concrete. Stowasser, U.S. Pat. No. 3,672,182 discloses injecting liquid nitrogen into the water which is to be mixed with the cement to form concrete.
U.S. Pat. Nos. 4,300,355 and 4,305,257 disclose in-line slush making processes. These references teach injecting a cryogenic fluid into a flowing liquid stream to form a flowable liquid-solid mixture. These references teach that these processes can be used to treat the water component in a concrete making process.
Additionally the aggregate may also be cooled. This is done by spraying water to effect evaporation cooling. This method cannot achieve sufficient cooling even in conjunction with chilled water or ice. In many cases therefore, an additional or alternate method of cooling may be required. In these cases, it may be desirable to cool the cement.
Previously, cement was cooled by the manufacturer using water jacketed vessels. This process often proved unsatisfactory however since the extent to which the cement can be cooled is limited. Additionally, attempts were made to cool the cement by injecting liquid nitrogen directly into the storage bin to fluidize the cement. This fluidization alternative however is not practical since a complex diffuser system is necessary to ensure fluidization. This requires major modifications to an existing system and a large capital investment. Also, because of channeling, there results a great variation in the cement temperature. This presents a logisitical problem to the batch plant operator in trying to meet concrete temperature specifications. Channeling along the sides of the bed along with the low temperature of the exiting gas also exposes the silo to cryogenic temperatures.
Since these processes often result in insufficient cooling, it becomes necessary for the concrete producer to provide additional cooling to meet his specific temperature requirement. U.S. Pat. No. 3,583,172 teaches a method of cooling the concrete by spraying or injecting a cryogenic liquid directly onto or into the concrete mixture while it is being mixed. This method, however, is inefficient and exposes the concrete and the truck itself to cryogenic temperatures.